Light micrograph of Nostoc sp., a filamentous blue-green algae or cyanobacterium. The strands of alga are in fact linear colonies of primitive, single-celled, prokaryotic organisms. They are more closely related to bacteria than to algae. Nostoc has the ability to fix atmospheric nitrogen, thereby playing a major role in soil fertility. It has a wide distribution and is commonly found in soil, and on rocks and plants. It also acts as the algal partner to a number of lichens. Blue green algae are considered to be exceedingly primitive organisms dating back to Precambrian times. Magnification x140 at 35mm.